<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
  <style>
    /* .box{
      width: 200px;
      height: 200px;
      background-color: red;
      clip-path: polygon(0 0, 200px 0, 0 200px);
    } */
    /* .box{
      width: 0px;
      height: 0px;
      border: 50px solid #000;
      border-top: 50px solid #000;
      border-right: 100px solid #000;
      border-bottom: 50px solid #000;
      border-left: 100px solid #000;
      border-top-color: transparent;
      border-right-color: transparent;
      border-bottom-color: transparent;
    } */
  
    .box{
      width: 0px;
      height: 0px;
      border: 50px solid #000;
      border-radius: 80%;
      border-top-color: transparent;
      border-right-color: transparent;
      border-bottom-color: transparent;
      /* clip-path: polygon(0 0, 100px 0, 50px 50px); */
    }
  </style>
</head>
<body>
  <div class="box"></div>
</body>
</html>